Transmission of digital data is well-known in the art. Such transmission may occur over wire-lined transmission paths or wireless transmission paths. Such wireless transmission paths include radio frequency and infrared (IR). For IR transmissions, a light transmitting diode is pulsed on and off to generate a light pulse, which is subsequently received by a light receiving diode. Such wireless IR communication requires that the receiving transmission diode be in the line of sight with the transmitting diode
A wide variety of circuits utilize infrared transmission paths to communicate data from one device to another. For example, remote controls for televisions, radios, amplifiers, etc. use IR transmission paths to transmit data from the remote controller to the particular device. Current infrared technology developments have enabled computers to utilize infrared transmission paths. For example, an IR communication path may be established between a computer's central processing unit (CPU) and a printer. Utilizing such an IR path is based on an iRDA transmission standard of 4 PPM (four pulse position modulation).
4 PPM incorporates 500 nSec time slots to obtain a four megabits per second data rate. Each of the 500 nSec times slots are divided into four sections; where a data pulse may resided in any one of the four sections. If the pulse resides in the first section of a time slot, the data pulse represents a binary value of 00, if the pulse resides in the second section of the time slot, the data pulse represents a binary value of 01; if the pulse resides in the third section, the data pulse represents a binary value of 10, and if the pulse resides in the fourth section, the data pulse represents a binary value of 11.
A difficulty arises with infrared transmission due to the wide dynamic range of the infrared signal being transmitted. For example, if the transmitting diode and receiving diode are in close proximity (less than 0.25 meters), the current through the receiving diode may be as much as 10 mAmps. If, however, the transmitting diode and receiving diode are a substantial distance apart (greater than 2 meters), the current through the receiving diode may be as small as a 100 nAmps.
When the current through the receiving diode is approximately 100 nAamps it can be difficult to detect when a pulse exists and in which section of a time slot it resides. Typically, to detect the presence of a pulse, a comparison circuit is to compare an incoming data signal, which has been amplified, to a predetermined threshold. As apparent, when the magnitude of the incoming data signal is large, it is easy to detect its. Conversely, when the magnitude of the incoming data signal is small, the ability to detect its presence becomes difficult, resulting in valid pulses not be detected.
One solution to more accurately detect small magnitude data signals (Eg. 100 nAmp data signals created by a light receiving diode) is an adaptive threshold circuit. The adaptive threshold circuit generates an adaptive threshold, which is a fixed threshold plus a representation of the magnitude of the data signal. As such, the adaptive threshold is based on the magnitude of the data signal regardless of the magnitude of the data signal. A difficulty arises with the adaptive threshold circuit, in that, when the magnitude of the data signal is very small, it is undetected because of the representation portion of the adaptive threshold. When pulses go undetected, the transmitted data is corrupted and unusable.
Currently, there are several infrared data detection circuits that operate off of a five-volt power supply. These circuits, however, are not designed to operate from, nor do they function when powered by, a low-voltage power supply, such as three-volts. In addition, the currently available infrared data detection circuits are very sensitive to variances in the characteristics of the light receiving diode. If the light receiving diode characteristics change considerably, many of the five-volt powered infrared data detection circuits will not function. Therefore, a need exists for a method and apparatus for an infrared data detection circuit that operates at low power and is relatively insensitive to changes in the light receiving diode characteristics.